The present invention relates to an imaging apparatus for picking up images at low light levels, and more particularly to an imaging apparatus that uses an electron-bombarded multiplying tube with an internal charge-transfer solid imaging device.
Technology that uses, as an imaging device, an electron-bombarded multiplying tube with an internal charge-transfer solid imaging device, such as a CCD imaging device, is disclosed in xe2x80x9cElectron-bombarded Back-illuminated CCD Sensors for Low Light Level Imaging Applicationsxe2x80x9d, SPIE Vol. 2415, pages 211-235, 1995 by G. M. Williams Jr. et al.
The above-noted publication discloses an imaging device with an internal Full Frame Transfer (FFT) CCD that uses almost the entire surface area of the CCD as the imaging region. In order to perform reading operations at television rates using an FFT CCD, incident light is blocked using a shutter, for example, during the vertical transfer period. Further, in order to match interlace scanning used in television, that is, in order to obtain image information corresponding to odd field image information, which is formed from image information from odd rows of television scan lines, and to obtain image information corresponding to even field image information, which is formed from image information from even rows of television scan lines, a horizontal transfer section combines image information in every two horizontal pixel rows of the CCD so that image information corresponding to the odd field and the even field can be alternately output.
Japanese Patent Publication (Rokoku) No. 60-30059 discloses technology that uses, as an imaging device, an electron-bombarded multiplying tube with an internal Frame Transfer (FT) CCD that is also capable of operating at television rates. FIG. 7 is a schematic view showing pixel formation by virtue of a CCD used in this technology. A transfer electrode 53 is formed in a surface opposite an incidence plane of an Si substrate 51. The transfer electrode 53 is configured from four electrodes 53a to 53d per one pixel. In order to perform the above-described interlace operations, a potential well 52 is formed below the transfer electrodes 53a, 53b during charge accumulation and retrieval of the even field and another potential well 52 is formed below the transfer electrodes 53c. 53d during charge accumulation and retrieval of the odd field, thereby performing quasi-interlace operations. Accordingly, the FT-CCD needs only half the number of pixels for the effective scanning lines.
However, the FFT-CCD disclosed by Williams Jr. et al requires incident light to be blocked during the transfer period. Therefore, the energy in light incident during this period cannot be used effectively. This is particularly a problem when incident light has a low intensity. When operating at television rates, accumulation time for imaging a single field is 1/60 seconds (17 ms) at maximum. However, this CCD requires about 12 ms to transfer one-field image. This leaves only about 5 ms for accumulating one-field image. This means that only about 30% of the incident light is used for accumulating the image. For this reason, sensitivity could not be increased. It is difficult to improve the transfer speed of the FFT-CCD while maintaining the S/N ratio and transfer efficiency at good levels, so there is a limit to how much the usage rate of light can be improved.
On the other hand, although the FT-CCD disclosed in Japanese Patent Publication No. 60-30059 has a signal usage rate of close to 100%, it has a disadvantage in that dark current of the CCD increases in association with electron beam irradiation. It is conceivable that this is because braking radiation X rays are generated at the electron incidence plane of the CCD when electrons emitted from the photocathode are accelerated to about 8 keV and irradiated onto the CCD, and that the X rays create an interface energy level at the silicon oxide film interface near the transfer electrode provided on the surface of the CCD.
As one method for reducing dark current in a CCD, U.S. Pat. No. 4,963,952 discloses a method of applying a voltage that is negative with respect to substrate potential, to the vertical transfer electrode during the signal accumulation period. However, the FT-CCD disclosed in the above-described Japanese Patent Publication No. 60-30059 applies a positive voltage to the gate electrode in the signal accumulation position during the accumulation period, in order to change accumulation position of the signal charge in each pixel when switching between the even and odd fields. Therefore, the PT-CCD of Japanese Patent Publication No. 60-30059 could not use the technology of U.S. Pat. No. 4,963,952. For this reason, the FT-CCD is greatly influenced by dark current resulting from X ray damage, and has a short life because of degradation caused by X rays at the interface. Also. CCD has only half the number of pixels in the vertical direction as a television scan line, so resolution was not sufficient.
In view of the foregoing, it is an object of the present invention to provide an imaging apparatus that uses an electron-bombarded multiplying tube with an internal solid imaging device with a long life, improved S/N ratio, high resolution, and moreover high sensitivity.
To achieve the above-described object, an imaging apparatus according to the present invention includes, as an imaging element, an electron-bombarded multiplying tube housing, in a vacuum vessel that partially transmits light, a photocathode that emits, in accordance with incident light, photo-electrons from a photo-electron emission surface opposite a light incidence plane, and a charge-transfer solid imaging device that is disposed in confrontation with the photo-electron emission surface of the photocathode and that detects spatial distribution of photoelectrons as an image using a plurality of pixels. The photocathode is applied with a voltage that is negative with respect to a substrate of the imaging device. The imaging apparatus is characterized in that the imaging device is configured from a plurality of pixels disposed in horizontal pixel rows in a main scanning direction, the horizontal pixel rows being disposed in a vertical direction that is orthogonal to the main scanning direction, in a number equal to or greater than a predetermined number of output scan lines, that the image device is configured from an imaging portion for multiplying and accumulating electrons falling incident on the pixels during an image accumulation period, and an accumulation portion for, during a successive horizontal transfer period, transferring to and accumulating in each corresponding pixel, charges accumulated in the imaging portion during the image accumulating period, and that a charge transfer electrode is formed on a surface opposite the photo-electron incidence plane on a substrate of the imaging device, and a voltage that is negative with respect to the substrate is applied to the transfer electrode of the imaging portion at least during the image accumulation period.
With this configuration, the back-illuminated FT solid imaging device is disposed in the vacuum vessel in confrontation with the photocathode. In the FT solid imaging device, charge accumulated during the image accumulation period can be transferred to the accumulation portion at a high speed during the charge transfer period. Therefore, the image accumulation period is sufficiently long. Also, a negative voltage is applied to the transfer electrode of the imaging portion during the image accumulation period. Therefore, dark current associated with incident brake radiation X rays can be reduced. Further, vertical resolution can be secured because the number of vertical pixels equal or exceed a predetermined number of output scan lines.
Further, the horizontal pixel rows are scanned sequentially in a vertical direction and an image signal that is equivalent to the accumulated charges at each pixel is output. A scan converter is further provided for converting the image signal into an interlace signal by alternately outputting one field at a time of only the odd rows or only the even rows of the horizontal pixel rows.
Alternatively, a horizontal transfer portion adds, at every other fields, an output signal for the 2n-th horizontal pixel row in the vertical direction to an output signal for the (2nxe2x88x921)th or the (2n+1)th horizontal pixel row in alternation, wherein n is a natural number.
With this configuration, the imaging portion and the accumulation portion of the imaging device are provided with the same form as a sequentially-output-operation imaging device, and in addition convert the output signal for the imaging elements into an interlace signal.
On the other hand, it is desirable to further provide a frame memory for accumulating a single frame""s worth of image signal from the imaging device, and a subtraction circuit for subtracting the output signal accumulated in the pixel frame memory from the corresponding output signal of the scan converter or the imaging device and outputting the result.
With this configuration, the difference between the pixel output accumulated in the frame memory and the output signal of the imaging device or the scan converter be easily obtained.
Further, it is desirable that the voltage value applied to the photocathode be 0V during the vertical transfer period, or be a negative voltage having an absolute value that is smaller than the voltage value applied during the image accumulation period.
With this configuration, the potential difference formed between the photoelectron incidence plane and the photocathode during the charge transfer period from the imaging portion to the accumulation portion of the imaging device is smaller than the potential difference during the image accumulation period. The acceleration of photo-electrons from the photocathode to the photo-electron incidence plane of the device is controlled during the charge transfer period, so that the energy of electrons that reach the photoelectron incidence plane of the device is reduced during the charge transfer period. Therefore, gain from collision of electrons can be reduced.